NullReferenceExeption in controller

Question

The error: My Controller where the exception is trown:

    public class UitgiftesController : Controller
    {
        private readonly ApplicationDbContext _context;

        private UserManager<Medewerker> _userManager;

        //class constructor
        public UitgiftesController(UserManager<Medewerker> userManager)
        {
            _userManager = userManager;
        }

        // GET: Uitgiftes
        public async Task<IActionResult> Index()
        {
            var applicationDbContext = _context.Uitgiftes.Include(u => u.Onderdelen);
            return View(await applicationDbContext.ToListAsync());
        }

        // GET: Uitgiftes/Details/5
        public async Task<IActionResult> Details(int? id)
        {
            if (id == null)
            {
                return NotFound();
            }

            var uitgiftes = await _context.Uitgiftes
                .Include(u => u.Onderdelen)
                .FirstOrDefaultAsync(m => m.Id == id);
            if (uitgiftes == null)
            {
                return NotFound();
            }

            return View(uitgiftes);
        }

        // GET: Uitgiftes/Create
        public IActionResult Create()
        {
            ViewData["OnderdelenId"] = new SelectList(_context.Set<Onderdelen>(), "Id", "Naam");
            return View();
        }

        // POST: Uitgiftes/Create
        // To protect from overposting attacks, enable the specific properties you want to bind to, for 
        // more details, see http://go.microsoft.com/fwlink/?LinkId=317598.
        [HttpPost]
        [ValidateAntiForgeryToken]
        public async Task<IActionResult> Create([Bind("Id,Tijdstip,GewichtKg,Prijs,OnderdelenapparaatId,OnderdelenId,MedewerkerId")] Uitgiftes uitgiftes)
        {
            if (ModelState.IsValid)
            {
                _context.Add(uitgiftes);
                await _context.SaveChangesAsync();
                return RedirectToAction(nameof(Index));
            }
            ViewData["OnderdelenId"] = new SelectList(_context.Set<Onderdelen>(), "Id", "Naam", uitgiftes.OnderdelenId);
            return View(uitgiftes);
        }

The Controller where the info comes from:

        private readonly ApplicationDbContext _context;

        public OnderdelensController(ApplicationDbContext context)
        {
            _context = context;
        }

        // GET: Onderdelens
        public async Task<IActionResult> Index()
        {
            return View(await _context.Onderdelen.ToListAsync());
        }

        // GET: Onderdelens/Details/5
        public async Task<IActionResult> Details(int? id)
        {
            if (id == null)
            {
                return NotFound();
            }

            var onderdelen = await _context.Onderdelen
                .FirstOrDefaultAsync(m => m.Id == id);
            if (onderdelen == null)
            {
                return NotFound();
            }

            return View(onderdelen);
        }

        // GET: Onderdelens/Create
        public IActionResult Create()
        {
            return View();
        }

        // POST: Onderdelens/Create
        // To protect from overposting attacks, enable the specific properties you want to bind to, for 
        // more details, see http://go.microsoft.com/fwlink/?LinkId=317598.
        [HttpPost]
        [ValidateAntiForgeryToken]
        public async Task<IActionResult> Create([Bind("Id,Naam,Omschrijving,VoorraadKg,PrijsPerKg")] Onderdelen onderdelen)
        {
            if (ModelState.IsValid)
            {
                _context.Add(onderdelen);
                await _context.SaveChangesAsync();
                return RedirectToAction(nameof(Index));
            }
            return View(onderdelen);
        }

I have set everything to null for onderdelen(components) in the database.

Have I not made the relations between them good?

            //Verbinding Many to Many Innameapparaat
            modelBuilder.Entity<Innameapparaat>()
                .HasKey(c => new { c.InnamensId, c.ApparaatId });

            modelBuilder.Entity<Innameapparaat>()
                .HasOne(pt => pt.Innames)
                .WithMany(p => p.Innameapparaats)
                .HasForeignKey(pt => pt.ApparaatId);

            modelBuilder.Entity<Innameapparaat>()
                .HasOne(pt => pt.Apparaten)
                .WithMany(p => p.Innameapparaats)
                .HasForeignKey(pt => pt.InnamensId);


            //Verbinding Many to Many Onderdeelapparaat
            modelBuilder.Entity<Onderdeelapparaat>()
                .HasKey(c => new { c.OnderdelenId, c.ApparaatId });

            modelBuilder.Entity<Onderdeelapparaat>()
                .HasOne(pt => pt.Onderdelen)
                .WithMany(p => p.Onderdeelapparaats)
                .HasForeignKey(pt => pt.ApparaatId);

            modelBuilder.Entity<Onderdeelapparaat>()
                .HasOne(pt => pt.Apparaten)
                .WithMany(p => p.Onderdeelapparaats)
                .HasForeignKey(pt => pt.OnderdelenId);
        }
        public DbSet<RobinGroot.Models.Uitgiftes> Uitgiftes { get; set; }
        public DbSet<RobinGroot.Models.Apparaten> Apparaten { get; set; }
        public DbSet<RobinGroot.Models.Innames> Innames { get; set; }
        public DbSet<RobinGroot.Models.Onderdelen> Onderdelen { get; set; }

I have searched for this problem but I could not solve the problem on my own so I could really use some help with this.

Answer 1

Fix your controller constructor:

private readonly ApplicationDbContext _context;

        private UserManager<Medewerker> _userManager;

        //class constructor
        public UitgiftesController(
                       UserManager<Medewerker> userManager,
                         ApplicationDbContext context
)
        {
            _userManager = userManager;
            _context=context;
        }

and your select list:

  ViewData["OnderdelenId"] = new SelectList(_context.Set<Onderdelen>().ToList(), "Id", "Naam");